When handling volatile liquids such as hydrocarbons including gasoline and kerosene, air-volatile liquid vapor mixtures are readily produced. The venting of such air-vapor mixtures directly into the atmosphere results in significant pollution of the environment and a fire or explosion hazard. Accordingly, existing environmental regulations require the control of such emissions.
As a consequence, a number of processes and apparatus have been developed and utilized to recover volatile liquids from air-volatile liquid vapor mixtures. Generally, the recovered volatile liquids are liquified and recombined with the volatile liquid from which they were vaporized thereby making the recovery process more economical.
The initial vapor recovery systems utilized in the United States in the late 1920's and early 1930's incorporated a process combining compression and condensation. Such systems were originally only utilized on gasoline storage tanks. It wasn't until the 1950's that local air pollution regulations began to be adopted forcing the installation of vapor recovery systems at truck loading terminals. Shortly thereafter, the "clean air" legislation activity of the 1960's, which culminated in the Clean Air Act of 1968, further focused nationwide attention on the gasoline vapor recovery problem. As a result a lean oil/absorption system was developed. This system dominated the marketplace for a short time.
Subsequently, in the late 1960's and early 1970's cryogenic refrigeration systems began gaining market acceptance (note, for example, U.S. Pat. No. 3,266,262 to Moragne). While reliable, cryogenic systems suffer from a number of shortcomings including high horsepower requirements. Further, such systems require relatively rigorous and expensive maintenance to function properly. Mechanical refrigeration systems also have practical limits with respect to the amount of cold that may be delivered, accordingly, the efficiency and capacity of such systems is limited. In contrast, liquid nitrogen cooling systems provide more cooling than is required and are prohibitively expensive to operate for this type of application.
As a result of these shortcomings, alternative technology was sought and adsorption/absorption vapor recovery systems were more recently developed. Such a system is disclosed in a number of U.S. Patents including, for example, U.S. Pat. No. 4,276,058 to Dinsmore, the disclosure of which is fully incorporated herein by reference. Such systems utilize a bed of solid adsorbent selected, for example, from silica gel, certain forms of porous mineral such as alumina and magnesia, and most preferably activated charcoal. These adsorbents have an affinity for volatile hydrocarbon liquids. Thus, as the air-hydrocarbon vapor mixture is passed through the bed, a major portion of the hydrocarbons contained in the mixture are adsorbed on the bed. The resulting residue gas stream comprising substantially hydrocarbon-free air is well within regulated allowable emission levels and is exhausted into the environment.
It should be appreciated that the bed of adsorbent used in these systems is only capable of adsorbing a certain amount of hydrocarbons before reaching capacity and becoming ineffective. Accordingly, the bed must be periodically regenerated to restore the carbon to a level where it will effectively adsorb hydrocarbons again. This regeneration of the adsorbent is a two step process.
The first step requires a reduction in the total pressure by pulling a vacuum on the bed that removes the largest amount of hydrocarbons. The second step is the addition of a purge air stream that passes through the bed. The purge air polishes the bed so as to remove substantially all of the previously adsorbed hydrocarbons. These hydrocarbons are then pumped to an absorber tower wherein an absorber fluid such as lean oil or other nonvolatile liquid solvent is provided in a countercurrent flow relative to the hydrocarbon rich air-hydrocarbon mixture being pumped from the bed. The absorber fluid condenses and removes the vast majority of the hydrocarbons from that mixture and the residue gas stream from the absorber tower is recycled to a second bed of adsorbent while the first bed completes regeneration.
In order to achieve the most effective and efficient recovery of hydrocarbon from the hydrocarbon rich air-hydrocarbon mixture, it is necessary to maintain a particular level of absorber fluid during absorber tower operation. In the past this has been accomplished using a level control valve and float assembly arrangement. While such an arrangement is effective for its intended purpose, it does suffer from a number of shortcomings.
First, it should be appreciated that dirt and rust, elements commonly found in the operating environment of most vapor recovery systems, tend to foul the operation of the level control valve and float assembly. Further, in cold weather conditions moisture and condensation contacting the level control valve and float assembly may freeze effectively preventing these components from properly operating and maintaining the necessary level of absorber fluid in the absorber tower to provide efficient operation.
Second, it should be appreciated that many state of the art float assemblies incorporate diaphragms made from a resilient material such as polytetrafluoroethylene. Unfortunately, this material stiffens in cold weather conditions thereby impairing proper function. Further, many hydrocarbon fuel additives in use today chemically attack the material from which the diaphragms are constructed thereby necessitating frequent maintenance intervals for replacement and repair (perhaps as often as quarterly in northern climates subject to greater temperature extremes).
Many other state of the art float assemblies incorporate piston arrangements in place of diaphragms. It should be appreciated, however, that these piston arrangements must include sealing rings. The same hydrocarbon fuel additives noted above for chemically attacking diaphragms, chemically attack the materials from which these piston rings are constructed. Such chemical attack often leads to ring swelling and disconfiguration that impairs proper float assembly operation. Accordingly this type of construction also nessitates frequent maintenance intervals for replacement and repair. The resulting downtime substantially reduces loading terminal productivity. Further, repairs may be unexpectedly required thereby interrupting delivery schedules and creating other significant inconveniences.
Third, it should be noted that the level control valve is a purely mechanical device and as such is subject to constant wear. In particular, most level control valves incorporate a needle valve that becomes worn over time. Eventually this wear leads to a leaking condition that necessitates repair and further down time.
A need is, therefore, identified for a new and improved approach for controlling the level of absorber fluid in the absorber tower during vapor recovery system operation.